Multi-beam multi-column electron beam inspection system

ABSTRACT

A multi-column electron beam inspection system is disclosed herein. The system is designed for electron beam inspection of semiconductor wafers with throughput high enough for in-line use. The system includes field emission electron sources, electrostatic electron optical columns, a wafer stage with six degrees of freedom of movement, and image storage and processing systems capable of handling multiple simultaneous image data streams. Each electron optical column is enhanced with an electron gun with redundant field emission sources, a voltage contrast plate to allow voltage contrast imaging of wafers, and an electron optical design for high efficiency secondary electron collection.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/183,724 filed Feb. 19, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to the field of wafer defect inspection,and in particular to wafer inspection using charged particle beams.

[0004] 2. Description of the Related Art

[0005] Defect inspection of semiconductor wafers and masks for ICmanufacturing is an accepted production process for yield enhancement.The information obtained from a wafer defect inspection tool can be usedto flag defective dies for repair, or to improve wafer processingparameters. The systems used for inspection are typically optical innature and such systems are limited in resolution due to the wavelengthof the measuring light (200-400 nm). Although possible, it becomesincreasingly difficult to detect defects that are smaller than thewavelength of the measuring light. Defects can range in both size andshape, and have been measured to be as small as one quarter the size ofthe critical dimension (CD) (Hendricks et. al., SPIE vol. 2439 pp.175-183, 1995). For example, in order to short two adjacent traces, aconductive shorting defect can be significantly thinner than the traces,and yet still cause the circuit to fail. As the feature sizes onsemiconductor wafers drops below 0.18 μm, conventional opticalinspection techniques will have difficulty detecting the smaller-sizeddefects.

[0006] To overcome this resolution limit, several electron beaminspection systems have been designed. The formation or creation of animage in an electron beam inspection system is similar to that of aScanning Electron Microscope (SEM). A primary beam of electrons from anelectron column is focused to a small spot on an object that is to beimaged (e.g. a silicon wafer containing microcircuits). The primary beamof electrons interacts with the object such that the object emitselectrons. The electrons emitted from the object are typicallyclassified into two groups: those with low energy (less than roughly 50eV) are called secondary electrons; those that are emitted with highenergy (close to the energy of the primary beam) are calledbackscattered electrons. Secondary electrons are emitted from a regionclose to the surface of the object and therefore give topographicalinformation. Backscattered electrons can come from deeper within theobject and typically give information about the atomic composition ofthe object, with only minor topographical information. Signalinformation is collected to form an image by detecting either secondary,backscattered or both types of electrons. An image is formed by scanningthe primary beam over the object in a raster fashion—such as in atelevision or cathode ray tube—and collecting thesecondary/backscattered electrons. The signal from the detectedelectrons is displayed synchronously with the primary beam raster on adisplay monitor, or stored on photographic film or computer memory.Magnification is achieved by the ratio of the size of the raster scan onthe object and the size of the display. The information from images canbe divided into small regions called pixels for digitization andcomputer manipulation. Typically the size of the focused electron beamis between one and two times the pixel size for optimum image quality.

[0007] In order to optimize the imaging process using an e-beam system,a knowledge of the materials is required. The object materials forimaging under an electron beam fall into two categories: conductive andinsulating materials. Special care must be used when imaging insulatorsas they charge under electron bombardment. In some cases charging of theinsulators can be so severe that no useful image can be obtained. Thecharging of materials occurs when there is an imbalance between thenumber of electrons striking the object and the number of electronsleaving the object. The number of electrons that leave the surface isdependent on the primary beam energy. The ratio of the number ofelectrons leaving the surface to the number of electrons hitting thesurface is typically larger than one when the primary beam is in theenergy range of 200-2000 eV. For insulating materials, when the numberof electrons leaving the surface is greater than the number entering thesurface, the surface charges positively until an equilibrium voltage isreached. When the primary beam energy is outside this range the numberof electrons leaving the surface is less than the number entering andthe surface charges negatively until an equilibrium voltage is reached.Therefore it is practical to image insulating materials in low voltagemode with the primary beam energy less than 2000 eV. Integrated circuitsor microcircuits are comprised of both conducting and insulatingmaterials and therefore low beam voltage operation is preferred.

[0008] Electron beam systems can have much higher resolution thanoptical systems because the wavelength of the electron can be in theangstrom regime. Present SEMs can have resolution down to 10 Å or evenless, operating at currents less than 1 nA. Electron beam systems arelimited in the speed at which they can inspect wafers. In presentsystems, throughputs of approximately 30 minutes per square cm have beenreported (Hendricks et al, SPIE vol. 2439, pg. 174). Thus to inspect anentire 300 mm diameter silicon wafer, approximately 70 hours will berequired. These systems can be used in a sampling mode where onlyseveral dies are inspected, thereby increasing throughput to severalhours per wafer. These systems have been effective for research andproduct development, but are impractical for volume production. Theseelectron beam inspection systems have a single electron beam that imagesthe wafer. There are two insurmountable problems for a single electroncolumn system to achieve sufficient wafer inspection throughput. First,space charge effects significantly reduce resolution when the electroncurrent is increased for adequate signal-to-noise ratios. Second, thedata rate from a single column is obtained serially from the electrondetectors within the column; as imaging resolution increases, this datarate cannot be handled by present-day image computers. An estimated10-20 Gbytes/s data rate is required.

[0009] An electron beam inspection system with high resolution andthroughputs of several wafers per hour could be used to inspect parts ofeach wafer, or perhaps to inspect one or more complete wafers out of acassette during the time required to complete a single processing stepfor all the wafers. Such an in-line wafer defect inspection tool wouldsignificantly increase the yield of ICs for chip manufacturers.

SUMMARY OF THE INVENTION

[0010] An electron beam inspection system is described which iscomprised of an electron optics assembly for generating multipleelectron beams, a voltage contrast plate to allow voltage contrastimaging of a wafer, multiple secondary electron detectors forsimultaneous imaging of a wafer using multiple electron beams, a waferstage with six degrees of freedom of movement, multiple image storagememory devices connected to the secondary electron detectors, multipleimage computers connected to the image storage memory devices, multiplepost-processors connected to the image computers, and a displayconnected to the multiple post-processors. Further embodiments of theinspection system have beveled edges to the openings in the voltagecontrast plate, so as to produce a field-free region at the wafer.Furthermore, the electron optics assembly comprises multiple electronbeam columns. In preferred embodiments the electron optics assembly iscomprised of electrostatic electron optical components. An electron beamcolumn is comprised of an electron gun, an accelerating region, ablanking aperture, deflectors, focus lenses, and a field-free tube. Inpreferred embodiments the electron gun comprises an array ofindividually operable field emission cathodes bonded to an array of beamlimiting apertures. Furthermore, the accelerating region may comprise ofan alignment deflector, accelerator plates and a shield electrode. Thedeflectors may comprise of a mainfield deflector and a sub-fielddeflector.

BRIEF DESCRIPTION OF THE FIGURES

[0011]FIG. 1 shows a top plan view schematic of a multi-column assemblyover a wafer; one electron beam per column is shown.

[0012]FIG. 2 shows a schematic cross-sectional representation of asingle column.

[0013]FIG. 3 illustrates an imaging strategy for a multi-column electronbeam inspection system.

[0014]FIG. 4A shows a schematic representation of a field emissionsource, as used in a multi-column electron beam inspection system.

[0015]FIG. 4B illustrates electron beam trajectories for the source ofFIG. 4A.

[0016]FIG. 5 shows a schematic representation of an array of fieldemission sources, as used in a multi-column electron beam inspectionsystem.

[0017]FIG. 6 shows a schematic cross-sectional representation ofaccelerating and deflection components of a multi-column electron beaminspection system.

[0018]FIG. 7 shows a schematic representation of a deflector assembly.

[0019]FIG. 8 shows a schematic cross-sectional representation offocusing optics.

[0020]FIG. 9 illustrates electron beam trajectories for the focusingoptics of FIG. 8.

[0021]FIG. 10 shows a schematic cross-sectional representation of asingle column with an annular secondary electron detector.

[0022]FIG. 11 shows a schematic cross-sectional representation of asingle column with a Wein filter.

[0023]FIG. 12 shows a schematic representation of a wafer stage.

[0024]FIG. 13 shows a schematic cross-sectional representation ofpumping apertures within a multi-column assembly.

[0025]FIG. 14 shows a schematic cross-sectional representation of thelower part of a multi-column assembly during plasma cleaning.

[0026]FIG. 15 shows a block diagram of a multi-column assembly and waferstage.

[0027]FIG. 16 shows a block diagram of a multi-column electron beaminspection system.

DETAILED DESCRIPTION

[0028] The invention disclosed herein is a high resolution electron beamwafer defect inspection tool that utilizes a multiple column, multiplesource assembly to achieve wafer inspection at high throughput (suitablefor in-line use in a semiconductor fabrication facility). The multiplecolumns allow for high throughput wafer inspection (up to several wafersper hour), and the multiple sources allow for both source redundancy,which will significantly decrease tool downtime and reduce servicecosts, and allow for more than one beam per column for advanced imaging.The tool is designed for the detection of defects for features withcritical dimensions (CDs) in the range 25 nm-100 nm.

[0029] The electron beam inspection tool has multiple electron opticalcolumns, each with its own electron source. There are no magneticcomponents in any of the columns—all of the focusing lenses and scanningdeflectors are electrostatic. FIG. 1 shows a schematic of a multi-columnassembly 100, positioned over a wafer 120. Such an assembly maytypically consist of 50-200 individual columns. Each column has anindividual electron beam 110. On hitting the wafer 120, each electronbeam 110, creates secondary electrons that are collected using electrondetectors located at the bottom of each column. The multiple columnapproach circumvents both of the primary problems that arise when usinga single column: space charge effects due to high electron current andhigh data rate from a single detector. Calculations using SIMION 3D, ver6.0 (a charged particle ray tracing program developed by David Dahl atthe Idaho National Engineering Lab) and MEBS (an electron beam analysissoftware package developed by Munro's Electron Beam Software Ltd.)indicate that high resolution (<25 nm) spot sizes can be achieved at therequired electron current levels for each column. Also by using multiplecolumns, the wafer can be simultaneously imaged by all of the columns,significantly increasing the wafer throughput. Thus, the overall highdata rate of roughly 20 Gbytes/s can be distributed among the electrondetectors of all the columns. For example, with 200 electron columns,each detector would only require a 100 Mbytes/s data rate, which isachievable with existing detector hardware and imaging computers.

[0030] In one embodiment of the present invention there are roughly 200columns distributed over a 300 mm diameter wafer. Each column covers afootprint of 20×20 mm in size over the wafer. In another embodiment ofthe present invention, this column footprint can be 40×40 mm,corresponding to approximately 56 columns over a 300 mm diameter wafer.In other embodiments, the column footprint can be any size in between20×20 mm and 40×40 mm, including rectangular shapes. The minimum size ofthe column footprint is primarily dependent on the mechanical sizelimitation necessary to make all of the electrical connections to eachcolumn. A large column footprint decreases the number of total columnsused, and therefore increases the bandwidth required by each electrondetector and imaging computer. Therefore, the maximum size of the columnis dependent on the maximum achievable data rate. Also, a larger columnfootprint requires an increased stage travel, as will be explainedbelow. The eventual size of the column footprint will be an optimizationof these parameters.

[0031] In a multiple column inspection system, many individual imageswill be collected in parallel. Each column will have the following: itsown source for generating electrons, lenses for focusing the electronbeam, a scanning mechanism for raster scanning the beam over the wafer,and a detector for collecting the secondary electron signal.

[0032] Inspection of a wafer for defect detection in IC manufacturing iscurrently performed in the following modes: array, die-to-die, anddie-to-data base modes. Defects can be detected by comparing an image ofthe location under inspection with an area that contains the samepattern information that is either generated from a data base oracquired from another region on the wafer. In the die-to-die mode thepattern information is acquired from the corresponding area of aneighboring die. In array mode the pattern information is acquired fromthe same die but from an area of a repeating pattern such as that of aDRAM. These comparisons are done with a specialized image computer.

[0033] A critical element of the multiple column approach will be thealignment of the images or data from each column, so that a comparisonof different areas on the wafer is performed with the same patterninformation. Ideally, all the columns would have a pitch or spacing thatwas equal to the die or reticle pitch or spacing. (A reticle field isthe area printed by a mask which can contain more than one die or chip.)In this case all the columns would be aligned with the same imageinformation. In the case where the column pitch is not equal to the dieor reticle field pitch, two approaches can be used. In one approach thecolumn could be moveable. Placement of the columns could be done withsmall motors and alignment information can be obtained by measurement ofthe die pitch. This measurement could be performed optically in either apre-inspection module during wafer loading or if optical measurement wasplaced in-situ with the columns and fine alignment could be done usingthe individual columns. Fine alignment needs to be at the level ofseveral pixels, because a mismatch of several pixels can yield a falseinterpretation of the data in the image computer. A second approachwould be to align the image data electronically in a computer memory. Inthis approach exact measurement of die alignment marks would have to beperformed in several steps by groups of columns. Each group of columnswould consist of the columns that do align with the die. For example, ifthe column spacing is 20 mm by 20 mm and the die are 15 mm by 15 mm thenthe columns align with the dies on a pitch of 60 mm×60 mm. In this casethere would be 4 groups of columns. After each group has obtained it'spositional information, then location pointers can be used in the dataacquisition to align common pattern images or data.

[0034] A schematic cross-sectional representation of a single column isshown in FIG. 2, including the source substrate 200, field emitting tips205, gate electrodes 210, focus electrode 215, beam limiting aperture220, electron source 222, electron gun 225, alignment deflector 230,accelerating region 235, accelerating plates 237, shield electrode 238,mainfield deflector 240, subfield deflector 245, scanning deflectors247, blanking aperture 250, focusing lens 1 255, focusing lens 2 260,field-free tube 265, secondary electron (SE) detectors 270, voltagecontrast plate 275, wafer 280 and electron trajectories 285. Each columnis roughly 160 mm in length, with the majority of the length representedby the accelerating region 235 between the electron gun 225 and thedeflectors 240 & 245. Only 3 out of the 100 electron sources 222 in thepreferred embodiment of the invention are shown in FIG. 2. The functionand operation of each component within the column will be discussed indetail below.

[0035] Referring to FIG. 2, each electron optical column can be brokeninto 3 main sections: (1) the electron gun 225, comprising a fieldemission source 222, focusing optics 215, and beam limiting aperture220; (2) the accelerating region 235 and scanning deflectors 247,comprising the alignment deflector 230, accelerating plates 237, shieldelectrode 238, mainfield deflector 240 and subfield deflector 245; and(3) the focusing electrodes comprising the focusing lens 1 255, focusinglens 2 260, field free tube 265, voltage contrast plate 275 and wafer280, and SE detectors 270. A simplified view of the column operation isas follows. The electron gun 225 creates a narrow, focused electronbeam. The beam limiting aperture 220 defines the electron beamsemi-angle which is the electron optical equivalent to the numericalaperture (N.A.) of a light optical system. The alignment deflector 230precisely steers the beam down the center of the column. The electronbeam is accelerated to high energy through the accelerating region 235.The focusing electrodes focus the beam to a small spot on the waferwhile the scanning deflectors 240 & 245 scan the beam over the wafer 280in a raster scan. The secondary electrons created by the primary beamare captured by the SE detectors 270. Topographical information isdetermined by the secondary emission yields of the features on the wafersurface. The signal from the SE detectors 270 is passed on to an imagingcomputer for image processing and defect detection.

[0036] Consider a 300 mm diameter wafer with a total of 201 columns (forthe preferred embodiment of the present invention) simultaneously imagethe surface of the wafer. Each column covers an approximately 20 mm×20mm square footprint on the surface of the wafer, and each column isrequired to image the entire 20×20 mm footprint. Thus, the entiresurface area of the wafer is covered by the 201 columns. In thepreferred embodiment there is one active electron beam per column.

[0037] An imaging strategy incorporating stage motion is shownschematically in FIG. 3, which shows a wafer 300, the imaging area 305for each column, the electron beam 310 from each column, an imagingstripe 315 imaged by a single electron beam, and the X-Y axes for thestage and scan motion 320 & 325. The column footprint is roughly 20×20mm in the preferred embodiment of the present invention, and is equal tothe imaging area 305 for each column. The entire wafer will becompletely imaged when one column has completed its scan of its imagingarea 305, because the entire wafer is being simultaneously imaged by all201 columns. The area imaged by a column is comprised of a series ofimaging stripes 315 where an imaging stripe is defined as the areascanned in the X-direction 325 by the electron beam during a singlestage travel in the Y-direction 320 and extends across the whole lengthof the column footprint. In the preferred embodiment of the presentinvention, the imaging stripe width 330 is 10 μm. In a preferredembodiment the width of each stripe is composed of 400 imaging pixels,where each pixel is 25 nm×25 nm, although the pixel size can be smalleror larger depending on the resolution required. As the beam is scannedacross this 10 μm stripe in the X-direction 325, the stage is slowlyscanned in the Y-direction 320. As the stage scans across the entirefootprint of the column, the resulting stripe is 20 mm long in theY-direction 320 and 10 μm wide in the X-direction 325. After completingone 20 mm pass across the imaging area 305, the stage steps the wafer300 by 10 μm in the X-direction 325, and travels back across the columnfootprint in the direction opposite to its first pass, as depicted inFIG. 3. This process is repeated until the entire 20 mm×20 mm area isimaged with approximately 2000 imaging stripes. The wafer stage motionis called a serpentine motion (back-and-forth, imaging in both scandirections), covering the full 20 mm square imaging area with about 2000imaging stripes. Since all of the columns are imaging at the same time,the time taken to cover one imaging area is also the time that it takesto image the entire 300 mm wafer.

[0038] The following sections describe the detailed operation of eachcomponent of the electron optical column, starting from the electron gunand traveling down to the wafer.

[0039] The position of the electron gun 225 within the electron opticalcolumn can be seen in FIG. 2. In standard terms, this is considered the“top” of the column (at the top of FIG. 2), and the wafer 280 is locatedat the “bottom” of the column (at the bottom of FIG. 2). Each column hasits own electron gun 225. In a preferred embodiment of the presentinvention, the electron gun 225 consists of an array of microfabricatedcold cathode emitters combined with a beam limiting aperture 220; theemitters and the beam limiting aperture 220 are microfabricated on theirown die, followed by a flip-chip bond that affixes the two die together.The details of the structure and fabrication of this embodiment of theelectron gun 225 are found in Parker et al. “Electron optics formulti-beam electron beam lithography tool” U.S. nonprovisional patentapplication filed Nov. 23, 2000, which is incorporated herein byreference.

[0040] To FIG. 4A shows a schematic representation of field emissionsource and FIG. 4B illustrates the electron trajectories for saidsource, as modeled using SIMION 3D, ver 6.0. FIGS. 4A & 4B show thesource substrate 400, field emission tip 405, gate electrode 410, focuselectrode 415, tip-to-gate insulator 420, gate-to-focus insulator 425and electron trajectories 430. The electron gun is composed of an arrayof such field emission tips. In the preferred embodiment only one tipwill be operational per column at any given time. Due to the possibleshort lifetime of these tips, an array of tips can be used to allow fora large amount of redundancy. This significantly increases the electrongun lifetime. The number of tips in an array can be as small as one (asingle tip), or as large as 10,000 (an array of 100×100 tips); each tiphas the property to be individually addressable. Each tip 405 willrequire a focusing electrode 415 and a beam limiting aperture, althoughthese do not need to be independently controlled for each tip on thesource. After the electrons are emitted from the tips, they are focusedinto a parallel beam by the integrated focusing electrode 415. Thiselectrode is microfabricated and self-aligned to the gate hole to ensureminimal beam deflection and aberrations. The beam limiting aperture 230selects a small portion of the total tip current and allows this to passdown the column, as shown in FIG. 2. The part of the electron currentthat passes through the beam limiting aperture 230 is referred to as the“beamlet”. One of the key requirements for the electron gun is the beamdivergence of this beamlet as it leaves the aperture. A beam divergenceof less than 0.050 half angle is required so that the beam does notincrease significantly in diameter as it passes through the scanningdeflectors 247. By minimizing beam size, beam aberrations from thecolumn optics are minimized. The beam limiting aperture 230 will be onthe order of 1-10 μm in size, and will typically be circular in shape.The current passing though each aperture down the column will be roughly10 to 100 nA. In a preferred embodiment, the electron optics is designedsuch that a majority of the current that passes through the beamlimiting aperture will reach the wafer 280.

[0041]FIG. 5 shows a schematic of a 4×5 array of the field emissiontips, indicating the tips 505, gate holes 510 and focus holes 515. Theremay be as few as one tip or as many as thousands, if an X-Y matrixaddressable scheme to individually address a tip is used. A single tipgun would require a lifetime significantly longer than the required meantime between servicing of the inspection tool, whereas in a gun withmultiple tips another tip could be activated if and when one tip were tofail. This would significantly increase the lifetime of the gun andincrease the mean time between failure (MTBF) for the inspection toolcontaining such a gun. The disadvantage of increasing the number of tipsis the increase in fabrication complexity and the larger number ofelectrical interconnects required. The alignment deflectors 230 (seeFIG. 2) will need to be re-adjusted as different tips are turned on, butthis should be a very quick process with a short tool downtime (expectedto be less than 5 minutes).

[0042] Field emission tips 500 (see FIG. 5) are typically smallmolybdenum cones. The cone is centered in a small extraction, or gatehole 505, and a potential difference is applied between the gateelectrode and cone in order to extract electrons from the cone by thefield emission mechanism. The I-V (current-voltage) characteristics ofthe device are generally in accord with the Fowler-Nordheim equation.Cold cathode emission has an energy spread, typically less than ±0.5 eV,which is required for the electron optical design of the column, inpreferred embodiments. Each tip will be operated using a currentfeedback circuit to regulate the emitted current, as described in Parkeret al. “Electron optics for multi-beam electron beam lithography tool”U.S. nonprovisional patent application filed Nov. 23, 2000, which isincorporated herein by reference. This will significantly improvecurrent stability in the electron beamlet. The current regulationcircuit can correct for current fluctuations with a bandwidth of roughly1 MHz. Some fluctuations can occur with a time constant on the order ofpicoseconds, and thus cannot be eliminated using the regulation circuit.These fast fluctuations can be significantly reduced or eliminated byusing a current pulsing process, such as that described in “Low VoltageField Emission of Electrons and Ions from Cold Metals” R. Olson, Ph.D.Thesis, The University of New Mexico, 1999. The current-voltage (I-V)characteristics of the field emission current can be monitored duringoperation. When the I-V characteristics degrade beyond a certainthreshold value for a particular source, that particular source can beturned off at the next convenient time (e.g., between inspection ofwafers) and another source can be activated as a replacement. Thisprocess should prevent the majority of source failures during theimaging of a wafer.

[0043] In a preferred embodiment of the present invention, the gun has a10×10 array of field emitters on an approximately 100 μmcenter-to-center spacing. The cones are made from molybdenum and areapproximately 1 μm tall within an approximately 1 μm diameter gate hole.The focus hole is 15 μm in diameter. There is a beam limiting aperturefor each cone, with each aperture being 3 μm in diameter and located 200μm away from the tips (along the emission axis). The gate electrodes areindividually addressable using an X-Y matrix addressing scheme, and thefocus and aperture electrodes are common to all of the sources. Typicaloperating base current from a cone is approximately 2 μl, with a typicalbeamlet current (passing through the beam limiting aperture) of 30 nA.The cones can be pulse conditioned to keep the beamlet current andangular emission stable. The pulse conditioning is applied throughoutthe imaging process; for example, the base current is pulsed to 1 mA fora duration of 1 microsecond with a repetition rate somewhere between 1Hz and 0.001 Hz.

[0044] In other embodiments of the present invention, the cone materialcan be different from molybdenum and may include: Pt, Ni, Ir, Re, Si, ormetallized Si. The size of the array can vary anywhere from 1×1 to100×100 or beyond, the only limitation being the electrical connectionsand physical size of the source. Also, different electron emitters canbe used as the electron source—a few requirements are compact size,feasibility of fabrication into an array, high brightness of electronemission, emission stability, and long lifetime. Examples of alternativesources include planar cathodes or single crystal tungsten thermalemitters. It is also possible to use a single electron source for eachcolumn, rather than an array, including sources that are notmicrofabricated, e.g. Schottky emitters.

[0045] In a further embodiment of the present invention, more than onesource per column could be emitting simultaneously, resulting in morethan one beamlet per column at the wafer. This has the potential toimprove the inspection tool throughput.

[0046] As shown in FIG. 2, the accelerating region 235 and scanningdeflector region 247 of the electron optical column represents the vastmajority of the length of the column, which is roughly 160 mm in apreferred embodiment of the invention. Due primarily to the small scaleof the column, all of the lenses, rotators, deflectors, blankers, etc.are electrostatic; no magnetic elements are used. Concerns with magneticelements include the complexities of the fabrication on such a smallscale, and magnetic screening of one column from the next in the closelypacked array of columns. Most of the column components areprecision-machined metals, insulating ceramics and conductive ceramics.Some of the more complex metal electrodes are screen printed ontoceramic; simpler electrodes maybe be brazed to ceramic. Standardmechanical and optical alignment techniques are utilized to ensure thatall components are properly situated.

[0047] Each electron source 222 within a column provides a narrow,parallel beamlet of electrons 285 that pass through the column. Thealignment deflector 230 steers the beamlet 285 down the column into theideal position for the lower column optics. This alignment deflector 230is an independent deflector for each column in the multi-columninspection tool. In the preferred embodiment of the present invention,the alignment deflector 230 is a set of double octupole deflectors. Thepurpose of the alignment deflector 230 is to make every source appear asif it is emitting directly on the optical axis of the column. Twodeflectors are required because both trajectory and position need to becorrected. The first deflector “pushes” the beamlet towards the centerof the optic axis, and the second deflector “pulls” the beamlet suchthat the trajectory of the beamlet is directly parallel to the opticalaxis. On-axis beams have significantly less aberrations when passingthrough lenses and deflectors.

[0048] The longest part of the column is an accelerating region 235 thataccelerates the electrons from roughly 100 eV to roughly 6000 eV. In thepreferred embodiment of the present invention, the accelerating region235 can be made up of an array of metal shims that extend over the wholearea of the wafer and accommodate the accelerating region for all of thecolumns simultaneously; this is shown schematically in FIG. 6, whichshows the metal shims 600, holes in the shims 610 through which theelectrons travel, source plate 620, deflector assembly 630 and waferchuck 640. An open system like this will also allow for better vacuumpumpout of the system, which is critical for the field emission sources.The shims 600 are typically metal, and can be made from beryllium copperor any other non-magnetic material. Typical column bore diameters areroughly 10 mm. The applied potential at each plate increases linearlyfrom 100 V to 6000 V. The accelerating column can also be made fromresistive ceramic in one piece. A linearly increasing potential isdesirable because it does not introduce lensing effects in the beam thatcould distort the beam shape. In other embodiments of the acceleratingregion, the metal shims can be replaced with thin, interwoven mesh withholes for the column bores. In a further embodiment, the metal shims canbe removed altogether, provided that a uniform accelerating field can beestablished, and if the deflection voltages can be suitably shieldedbetween adjacent columns.

[0049] As seen in FIG. 2, after passing through the accelerating region235, the electrons pass through the shield electrode 238 and themainfield and subfield scanning deflectors 240 & 245. The shieldelectrode 238 is the last electrode in the accelerating region 235 anddefines the start of the deflector region. It is typically made of ametal, metallized ceramic or conducting ceramic plate. In the preferredembodiment of the present invention, the shield electrode 238 definesthe final voltage of the accelerating region, and the dc voltage atwhich the deflectors 247 are operated. In another embodiment, the shieldelectrode is eliminated and the final voltage of the accelerating region235 is defined by the mainfield deflector 240.

[0050] In a preferred embodiment, the mainfield deflector 240 iscomposed of a set of double deflection octupoles, requiring 16electrical connections. A deflector assembly is shown schematically inFIG. 7, comprising a mainfield deflector 700, a subfield deflector 710,insulating ceramic 720, and showing the column bore 730. The bore 730 istypically 2 mm in diameter. In a preferred embodiment, the mainfielddeflector 700 is fabricated using Ti alloy electrodes brazed to ceramic.Machining of the column bore 730 is done after brazing, so as to ensureconcentricity of both octupole deflectors that comprise the mainfielddeflector 700. Two octupole deflectors are used in order to ensure thatthe beamlet is telecentrically scanned on the wafer. Telecentricscanning means that the beamlet hits the wafer perpendicular to thewafer surface, and this allows for a larger depth of field of thefocused beam at the wafer.

[0051] The mainfield deflectors have two functions: (1) tracking thestage motion as it moves the wafer during imaging, and (2) performing alarge area scan to find alignment marks. The mainfield deflectors have atotal deflection capability of approximately ±50 μm in both the X and Ydirections on the wafer. However, at the edges of this scanfield, theaberrations in the beam are significant, and beam resolution and spotsize will be affected. Modeling of beam resolution and spot size hasbeen carried out using SIMION 3D, ver. 6.0 and MEBS software. Thesecalculations indicate that the imaging field, within which the spot sizeis sufficiently small to image a single pixel area (25×25 nm), isapproximately ±10 μm in X and Y for the mainfield deflectors.

[0052] In its second function, the mainfield deflectors are used tosearch for alignment marks on the wafer. This is achieved by scanningthe full scan field (±50 μm square) on the wafer to find the alignmentmarks. Global positioning should be sufficiently accurate to place thealignment marks within this scan field. Although the resolution or spotsize of the beamlet may not be as small as is needed for imaging, itwill be sufficient for the location of the alignment marks. Thealignment marks are imaged using the secondary electrons that areemitted from the wafer surface and are collected by the secondaryelectron (SE) detectors. Typical alignment mark metals can be, but arenot limited to, gold, tungsten, and titanium-tungsten.

[0053] The bandwidth of the applied voltage signal for the mainfielddeflectors is determined by the desired scan speed in order to find thealignment marks on the wafer. Typically, a bandwidth of roughly 50 kHzis more than sufficient for the mainfield deflectors.

[0054] In a preferred embodiment of the invention, the subfielddeflector 710 is a quadrupole deflector with 4 independent electrodes.In another embodiment, an octupole deflector could be used. Because anoctupole deflector has double the number of connections as a quadrupoledeflector, an octupole deflector would only be used if aberrations usinga quadrupole deflector resulted in increased spot size. The subfielddeflector 710 only requires a single deflector because it is locatedcloser to the back focal plane of the focusing lenses. Therefore,telecentric scanning can be achieved with only a single deflector. Asubfield deflector 710 is fabricated in a similar fashion to a mainfielddeflector 700.

[0055] The subfield deflector is used to scan the beamlet over the waferduring the imaging process. A typical scan distance on the wafer is ±5μm. This results in a 10 μm wide stripe imaged on the wafer (see FIG.3). After imaging a single stripe, the subfield deflectors quickly bringthe beamlet back to the starting point to scan again.

[0056] A schematic cross-sectional representation of the focusingelectron optics is shown in FIG. 8, comprising focus lens 1 800, focuslens 2 810, field-free tube 820, voltage contrast plate 830 wafer 840,and beamlet 850. The primary function of the focusing optics is to focusthe beamlet 850 to a small spot on the wafer 850. In a preferredembodiment this is achieved using simple cylindrical lenses with a borediameter of roughly 2 mm. The strongest focusing occurs between focuslens 2 810 and the field-free tube 820, as this is the location of thehighest electric field. Initial calculations using SIMION 3D, ver 6.0indicate that typical potentials applied to the focus elements are asfollows: focus lens 1=6,000 V, focus lens 2=7,200 V, field-free tube=980V. The potential on focus lens 2 is the primary adjustment used forobtaining a focused spot on the wafer. In a preferred embodiment, focuslens 2 is independently controlled for each column, while focus lens 1and the field-free tube are common to all of the columns. In a preferredembodiment these electron optical elements can all be made out of anyvacuum compatible, non-magnetic metal.

[0057] The field-free tube 820 is held at a potential that is slightlylower than that of the wafer 840. In a preferred embodiment, the wafer840 is held at 1 kV and the field-free tube 820 is held at 980 V. Thepurpose of the Field-Free Tube is to separate the secondary electrons(emitted from the wafer surface) from the primary electrons (in thebeamlet). Most secondary electrons have an energy range that is lessthan 10 eV relative to the wafer, depending on the surface material ofthe wafer, the incoming primary electron energy, etc. As the secondaryelectrons are emitted from the surface, they travel towards thefield-free tube 820. However, the secondary electrons have insufficientenergy to enter the tube because of the negative 20 V differentialbetween the field-free tube 820 and the wafer 840. Thus, the secondaryelectrons will be pushed away from the field-free tube 820. FIG. 9 showsthe modeled trajectories of both the primary electrons and the secondaryelectrons as calculated by SIMION 3D, ver 6.0. FIG. 9 shows the afield-free tube 900, SE detectors 910, voltage contrast plate 920, wafer930, primary electron trajectories 940 and secondary electrontrajectories 950. As can be seen, the primary electrons are focused downto a small spot on the wafer, and the secondary electrons are emittedfrom the surface of this wafer at this point. The secondary electronsare emitted in roughly a Lambertian angular distribution (cos θ). A fewof the secondary electrons return to the wafer, but the vast majority ofthe electrons travel up towards the field-free tube 900. As thesecondary electrons reach the edge of the field-free tube 900, theelectric field generated by the SE detectors 910 (held at roughly 5 kV),cause the secondary electrons to be accelerated towards the detectors910. The secondary electron collection efficiency of the SE detectors910 using this electron optical design has been calculated using SIMION3D, ver 6.0 to be approximately 80% of all secondary electrons emittedfrom the surface of the wafer.

[0058] The voltage contrast plate 920, shown in FIG. 9 to be positioneddirectly above the wafer 930, has two functions: (1) enhance secondaryelectron collection efficiency and (2) allow for voltage contrastimaging of the wafer surface. The voltage contrast plate 920 is held avery short distance above the wafer 930 (roughly 100 μm in the preferredembodiment). This spacing can be maintained by using an electrostaticchuck to hold the wafer down flat, and by using several lasertriangulators on the wafer stage to allow for correction of the waferposition relative to the voltage contrast plate in six degrees offreedom. The voltage contrast plate 920 has a small (roughly 1 mm) holethrough which the primary and secondary electrons travel. The sides ofthe hole are beveled at an angle of arctan(0.5) with respect to thewafer surface to assist in creating a field-free region on the waferwhen the voltage contrast plate is held at a potential very close to thewafer (typically about 995 V if the wafer is at 1 kV). This field-freeregion allows the secondary electrons to escape the surface moreefficiently as they travel towards the SE detectors. The voltage appliedto the voltage contrast plate can be adjusted so as to force low energysecondary electrons back to the wafer surface. If the secondaryelectrons have sufficient energy, then they will be able to escape thisfield and reach the SE detectors; however, if the secondary electrons donot have sufficient energy, then they will return to the wafer. Thus,the voltage contrast plate acts as an electron energy high pass filter,blocking low energy electrons and allowing high energy electrons to bedetected. The potential can be adjusted so that the high pass electronenergy filter can be tuned. Changing the voltage contrast platepotential results in small changes in the focusing of the primary beam,but this can be compensated for by using the main focusing optics tomaintain a small spot size.

[0059] The SE detector 910, shown in FIG. 9 can be a standard electrondetector. This detector 910 is positioned between the field-free tubeand the wafer 930, and is typically either a uniform annular detector,or a multi-sectored detector. A multi-sectored detector can detecttopographical information by adding angular information to the detectedsignal. An example of contrast due to angular variation in the secondarysignal is shading. Shading occurs because three dimensional features canblock trajectories of emitted electrons over the angular range where theobstruction occurs. In a preferred embodiment of the present invention,the SE detector 910 is a four-quadrant detector; this detector 910 isheld at a potential of roughly 5 kV, which allows for efficient signalgain. The secondary electrons are emitted from the wafer 930 in a fieldfree region towards the column. Because the field-free tube 900 at theend of the column is held at a potential lower than the wafer 930, theelectrons will be repelled from the field-free tube 900 and swepttowards the SE detectors 910. This efficiently separates the secondaryelectrons from the primary electrons. With a state of the art solidstate detector, calculations indicate that the imaging bandwidth can beroughly 100 MHz or higher.

[0060]FIG. 10 and FIG. 11 show further embodiments, with different typesof SE detector with different positions in the electron optical column.FIG. 10 shows mainfield deflector 1010, subfield deflector 1020,blanking aperture 1030, focusing lens 1 1040, focusing lens 2 1050,field-free tube 1060, annular SE detector 1070, voltage contrast plate1080, wafer 1090 and beamlet 1095. The SE detector 1070 is positionedwithin the bore of the optical column, and is annular. The voltageapplied to the field-free tube 1060 will need to be adjusted so thatsecondary electrons can penetrate the column bore and reach the SEdetector 1070. FIG. 11 shows mainfield deflector 1110, subfielddeflector 1120, blanking aperture 1130, focusing lens 1 1140, focusinglens 2 1150, field-free tube 160, SE detector 1170, voltage contrastplate 1180, wafer 1190, beamlet 1194, Wein filter 1196 and secondaryelectrons 1198. The SE detector 1170 is positioned off to the side ofthe column. In this approach, a magnetic lens (Wein filter 1196) isrequired to separate the secondary electrons 1198 from the primaryelectrons (beamlet 1194).

[0061] The wafer stage is a critical component in a wafer inspectionsystem, and is described in U.S. patent application Ser. No. 09/543,265entitled “Precision Stage” filed 04/05/2000, herein incorporated byreference. For a multi-column electron beam inspection system it iscritical that the stage be able to position the wafer so that it isparallel to the voltage contrast plate (positioning the wafer correctlyfor all imaging columns simultaneously); this requires a stage with 6degrees of freedom of movement. Such a stage is schematically shown inFIG. 12 comprising electrostatic chuck 1210, mirror plates 1220, magnets1230, X-Y linear motor coils 1240, legs 1250, flex joints 1260, andZ-axis actuators 1270. The stage needs to accurately position the waferto within several nanometers so that image data can be compared withouterror. Positional measurements are made with laser interferometers thatare accurate to sub-nanometer resolution. This information is used in afeedback loop to the stage controller to position the stage accurately.Another function of the stage is to perform the slow axis movement, asshown in FIG. 3.

[0062] Another requirement for the stage is low vibration and goodvibration isolation. Because a multicolumn system does not require thestage to move over the entire wafer but only over small areas of severaldies, other types of stages than the conventional roller bearing typecan be used. Roller bearing stages have an inherent problem of smallvibrations due to the ball bearings not being perfectly round and theways not being perfectly flat. The use of flexure joints 1260 in thestage allows for a smooth low vibration motion. Vibration isolation canbe controlled in the lateral direction by the use of force motors(magnets 1230 and coils 1240).

[0063] Another requirement for the stage is to be able to position thewafer extremely close to the electron optical components. A very shortworking distance improves lens aberrations, which limit the spot size,and aids in the collection efficiency of the secondary electrons. Thekey to achieving a close working distance is to keep the wafer flatusing an electrostatic chuck 1210 to clamp the wafer and the use of ameasurement and positional system that can measure and position thewafer to 6 degrees of freedom (DoF). With this type of stage control itis practical to maintain a working distance of 0.1 mm or smaller.

[0064] With the gap between the wafer and the columns being small,special care must to be taken to vacuum pump this region. Specialpumping holes may be necessary to insure that any possible pressure risedue to outgassing (the release of gas molecules from the wafer) isminimized. FIG. 13 shows focus lens 1 1310, focus lens 2 1320,field-free tubes 1330, SE detectors 1340, voltage contrast plate 1350,wafer 1360 and pump holes 1370.

[0065] The stage and column may also be configured to allow for plasmacleaning of the lower electron optical elements of the multiple columns,as shown in FIG. 14. FIG. 14 shows field-free tubes 1410, SE detectors1420, voltage contrast plate 1430, wafer 1440, plasma 1450 and powersupply and controller. Outgassing from wafers can contaminate theelectron optical elements. This contamination can result in unwanteddistortions to the electron beam. In a conventional SEM, apertures andparts of the column are cleaned on a periodic basis. The advantage ofplasma cleaning is that is does not require disassembly of the systembut can be done in-situ by applying sufficient voltage across the gapbetween the wafer 1440 and the voltage contrast plate 1430, while thevacuum is filled with a partial pressure of an active gas, such ashydrogen.

[0066]FIG. 15 shows a block diagram of the different components within apreferred embodiment of the present invention in terms of the electronoptics assembly and stage. As can be seen, the source, alignmentdeflectors, mainfield deflectors, subfield deflectors, field-free tubesand SE detectors are independent for each column. The acceleratingcolumn, focus lens 1, focus lens 2, and voltage contrast plate arecommon for all columns. The wafer sits on a wafer stage that iscarefully monitored and controlled using laser triangulators and laserinterferometers. Vacuum pumps maintain a vacuum pressure in the range of10⁻⁷ to 10⁻⁸ Torr at the wafer, and a pressure that is roughly one totwo orders of magnitude lower at the sources (this pressure differencecan be maintained by using differential pumping apertures in eachcolumn).

[0067] An inspection system can be broken down into several keycomponents: an imaging system to acquire pattern data for patternanalysis, a stage to move the wafer under the image acquiring system, amemory system for storing the data, an image computer which analyzes thepattern data, a post processor, a control computer for coordinating theinspection with all the other elements of the system, electronics todrive the system, a vacuum system required for electron beams, and awafer loading and handling mechanism. The components in a preferredembodiment of the present invention are shown in FIG. 16. Note that theduplication of components for this system is such that N is greater thanor equal to M, and M is greater than or equal to K.

[0068] In a wafer inspection, the image data is analyzed by a customizedimage computer which allows for flexibility in data format. One of theflexible elements is that the slow axis of the raster scan (see FIG. 3)can be performed with the stage. With this strategy, the image data canbe acquired in “swaths” and stored in computer memory. The height of theswath is determined by the strength of the deflectors which deflect thebeam electrostatically, and the length of the swath is the length of thestage travel. The length of the swath is typically determined by theamount of available computer memory.

[0069] After the data has been collected, the information is transferredto an image computer that analyzes the data using pre-programmedalgorithms. The simplest of these algorithms subtracts the data from twopixels that have the same pattern information. If there is a differencebetween the values of these two pixels, and this difference is greaterthan some parameter or threshold, then that pixel is determined to be adefect. All the defect information is then sent to a post processorwhich maps the defects by location and classifies them by type. Thisdefect classification can be performed manually by an operator or in anautomatic mode by using a defect analysis computer.

[0070] In other embodiments of the present invention, the column spacingcould be adjustable, with the columns positioned such that each columnwould inspect a single die. This would simplify the image processing interms of data stitching between adjacent columns; however, this wouldgreatly increase the mechanical complexity of the column design sinceeach column would need to be positioned accurately to within roughly 5μm (the scan size of the mainfield deflectors). If this approach isfollowed, a series of piezoelectric inchworms could be mounted betweeneach row and column. In another embodiment, the alignment deflectorscould be fabricated directly on the electron gun assembly rather than asa separate component within the accelerating region. In a furtherembodiment, the subfield deflectors could be octupole or dodecapoledeflectors rather than quadrupole deflectors. In another embodiment, thefield-free tube could be replaced with a grid to allow for improvedvoltage contrast adjustment. In a further embodiment, the focusingsystem could be improved by using fewer or more focusing lenses. Fewerfocusing elements would simplify the mechanical design, while morefocusing elements could improve spot size and depth of field. In anotherembodiment, it may be possible to improve upon the secondary electroncollection efficiency by using lenses of different shapes (rather thansimple cylindrical lenses). In a further embodiment, the wafer couldrange in voltage from 200 V-2,000 V rather than being fixed at 1 kV. Inanother embodiment, the use of magnetic lenses and deflectors could beintroduced. In a further embodiment, a snorkel lens, which is a magneticlens that resides below the wafer and causes the electrons to spiral upthe field lines towards a detector, could be used to aid in secondaryelectron collection efficiency. In another embodiment, the detectorcould have 1, 2, 3, or more sectors. In a further embodiment the beamlimiting apertures could be fabricated by conventional machining (asopposed to micromachining); furthermore, in some embodiments theelectron optical columns could function satisfactorily without beamlimiting apertures.

What is claimed is:
 1. An electron beam inspection system comprising: anelectron optics assembly; a voltage contrast plate; a multiplicity ofsecondary electron detectors situated between the electron opticsassembly and the voltage contrast plate; a wafer stage situated belowthe voltage contrast plate; a multiplicity of image storage memorydevices connected to the multiplicity of secondary electron detectors; amultiplicity of image computers connected to the multiplicity of imagestorage devices; a multiplicity of post-processors connected to themultiplicity of image computers; and a display connected to themultiplicity of post-processors.
 2. An electron beam inspection systemas in claim 1 wherein the wafer stage has six degrees of freedom ofmovement.
 3. An electron beam inspection system as in claim 1 furthercomprising a wafer situated on the wafer stage.
 4. An electron beaminspection system as in claim 3 wherein the voltage contrast plate hasopenings which are beveled at an angle so as to produce an electricfield free region at the wafer.
 5. An electron beam inspection system asin claim 1 wherein the electron optics assembly comprises multipleelectron beam columns.
 6. An electron beam inspection system as in claim5 wherein each electron beam column comprises: an electron gun; anaccelerating region situated below the electron gun; deflectors situatedbelow the accelerating region; a blanking aperture situated below thedeflectors; focus lenses situated below the blanking aperture; and afield-free tube situated below the focus lenses.
 7. An electron beaminspection system as in claim 1 wherein the electron optics assembly iscomprised of electrostatic electron optical elements.
 8. An electronbeam column comprising: an electron gun; an accelerating region situatedbelow the electron gun; deflectors situated below the acceleratingregion; a blanking aperture situated below the deflectors; focus lensessituated below the blanking aperture; a field-free tube situated belowthe focus lenses; a voltage contrast plate situated below the field-freetube; and a secondary electron detector situated between the field-freetube and the voltage contrast plate.
 9. An electron beam column as inclaim 8 wherein the electron gun comprises: an array of field emissioncathodes; and an array of beam limiting apertures bonded to the array offield emission cathodes.
 10. An electron beam column as in claim 9wherein the array of field emission cathodes are individually operable.11. An electron beam column as in claim 8 wherein the acceleratingregion comprises: an alignment deflector; accelerator plates situatedbelow the alignment deflector; and a shield electrode situated below theaccelerator plates.
 12. An electron beam column as in claim 8 whereinthe deflectors comprise: a mainfield deflector; and a subfield deflectorsituated below the mainfield deflector.
 13. An electron beam column asin claim 8 wherein the voltage contrast plate has openings which arebeveled at an angle so as to produce an electric field free region inthe vicinity of the opening.